1. Field of the Invention
This invention relates to a four wheel drive system for driving both front wheels and rear wheels by virtue of a power plant.
2. Description of Related Art
In a four wheel drive vehicle, it has been known to provide a cut-off clutch between the power plant and either the front wheel driving system or the rear wheel driving system. The cut-off clutch is disengaged to establish a two wheel driving system when the four wheel drive condition is not necessary to be maintained so that a power loss is reduced to improve fuel consumption efficiency.
Japanese Patent Public Disclosure No. 62-181916, laid open to the public in 1987, proposes a four wheel drive system in which a driving power is transmitted from the power plant to the rear wheels. The proposed four wheel drive system is provided with hydraulic clutches (hereinafter referred to as wheel clutches) on a rear wheel driving mechanism for the right and left rear wheels, respectively, to control driving torque for the right and left rear wheels. As a result, torque distribution between the right and left rear wheels is controlled to improve controllability of a vehicle in a steering condition.
It should, however, be noted that it to distribute the driving torque between the right and left rear wheel is difficult, specifically, under a steering condition or cornering condition of the vehicle.